(1) Field of the Invention
The present invention relates to a thickness-reduced deep-draw-formed can prepared from a resin-coated surface-treated steel plate. More particularly, the invention relates to a thickness-reduced deep-draw-formed can that exhibits excellent adhesiveness to an organic resin coating and excellent corrosion resistance, and has improved formability and form workability.
(2) Description of the Related Art
A process for producing a side-seamless can has heretofore been known comprising forming a metal blank such as an aluminum plate, a tin plate or a tin-free steel plate into a cup having a barrel with no seam on the side surface and a bottom integrally connected to the barrel without seam by subjecting the metal blank to at least one stage of drawing between a drawing die and a punch, and as required ironing the barrel of the cup between an ironing punch and an ironing die to reduce the thickness of the barrel. In preparing the side-seamless cans, it has also been known to use the metal blank on which is laminated a film of a thermoplastic resin such as polypropylene or a thermoplastic polyester.
In Japanese Unexamined Patent Publication No. 1-258822, the present inventors have proposed a process for reducing the thickness of the side wall of a can by bending and elongation at the time of deep-draw-forming the can. That is, the present inventors have proposed a redrawing process comprising holding a preliminarily drawn cup of a coated metal plate by an annular holding member inserted in the cup and a redrawing die, and relatively moving in mesh with each other the redrawing die and a redrawing punch that is provided coaxially with the holding member and the redrawing die in a manner to go into and come out from the holding member, in order to deep-draw-form a cup having a diameter smaller than that of the preliminarily drawn cup, wherein the radius of curvature (R.sub.D) of the working corner portion of the redrawing die is set to be 1 to 2.9 times as great as the blank thickness (t.sub.B) of the metal plate, the radius of curvature (R.sub.H) of the holding corner portion of the holding member is set to be 4.1 to 12 times as great as the blank thickness (t.sub.B) of the metal plate, flat portions of the holding member and the redrawing die engaging with the preliminarily drawn cup have a dynamic coefficient of friction of from 0.001 to 0.2, the draw-forming of at least one stage is carried out so that the redraw ratio defined by the ratio of the diameter of the shallow-draw-formed cup to the diameter of the deep-draw-formed cup lies in the range of from 1.1 to 1.5, and the side wall of the cup is uniformly bent to reduce the thickness along the entire direction of height thereof. As the coated metal plate, furthermore, it has been proposed to use a tin-free steel plate (electrolytically chromate-treated steel plate) coated with an epoxy-type paint.